The principal objective of this work is to add to our knowledge of the functional aspects of the vertebrate visual system. The approach is to study how the retinas of aquatic animals differ from those of animals which see in air. The differences are both structural and functional. These differnces are interpreted in terms of the physical differences in transmission of optical images in air and water. BIBLIOGRAPHIC REFERENCES: Easter, S.S. (1975) Vision Res. 15, 405-409. The time course of saccadic eye movements in goldfish. Easter, S.S. (1975) In Vision in Fishes (Ed. M.A. Ali), Plenum Press: New York, pp. 397-407. Horizontal eye movements in fish.